There are some good additional resources referenced in the docs here: https://response.pagerduty.com/resources/reading/

Specifically, Google's SRE books are particularly useful (https://landing.google.com/sre/books/) along with the book "Incident Management for Operations" (http://shop.oreilly.com/product/0636920036159.do) and Etsy's Debriefing Facilitation Guide (http://extfiles.etsy.com/DebriefingFacilitationGuide.pdf).

The book "Comparative Emergency Management" (https://training.fema.gov/hiedu/aemrc/booksdownload/compemmg...) is also quite interesting, as it compares the emergency management practices of about 30 different countries.

Firefighter for over 10 years and an IT career spanning Red Team/Blue Team and now SRE, I can't recommend enough taking the FEMA Independent Study course in Incident Command.

https://training.fema.gov/is/courseoverview.aspx?code=IS-100...

Even if you don't adopt the system it will help you frame and understand how complex IR can be. Having a scalable system to a) grow with resources and b) grow with external interactions is crucial to have BEFORE you need it.

Atlassian's handbook is pretty good: https://www.atlassian.com/software/jira/ops/handbook